Fix AbstractStringBuilder gaining extra APIs.

AbstractStringBuilder showed up in the API as gaining methods originally
belonging to StringBuffer/StringBuilder (mostly append(*) and
insert(*)).

Ideally it should not be in the API at all, but it's traditionally been
there and we'll keep it for now.

@hide the affected methods and move the "implements Appendable"
definition to StringBuffer/StringBuilder (as methods implementing an
interface can't be @hidden).

Bug: 27582395

(cherry picked from commit 616453db)

Change-Id: Ic5f5b4abf57dbde2a5ced09503f736258ff0fa72